1. Field of the Invention
The present invention relates in general to delta sigma modulators and in particular, to delta-sigma modulation circuits and methods utilizing multiple noise attenuation bands and data converters using the same.
2. Background of the Invention
Delta-sigma modulators are particularly useful in digital to analog and analog to digital converters (DACs and ADCS). Using oversampling, the delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
The typical delta sigma modulator includes a summer summing the input signal with negative feedback, a linear filter, quantizer and a feedback loop coupling the quantizer output and the inverting input of the summer. In a first order modulator, the linear filter comprises a single integrator or other filter stage while the loop filter in a higher order modulator comprises a cascade of a corresponding number of filter stages. Higher-order modulators have improved quantization noise transfer characteristics over those of lower order, but stability becomes a more critical design factor as the order increases. The quantizer can be either a one-bit or a multiple-bit quantizer.
Switched-capacitor filters/integrators are useful in a number of applications including the integrator stages in delta sigma modulators. Generally, a basic differential switched-capacitor integrator samples the input signal onto sampling capacitors during the sampling (charging) phase. A reference voltage may also be sampled onto a reference sampling capacitor during this phase to implement a DAC function in the feedback loop of an ADC. During the following dump phase, the charge on the sampling capacitor is transferred to the summing node of an operational amplifier and an integrator capacitor in the amplifier feedback loop. The operational amplifier drives the integrator output.
One drawback with switched-capacitor filters, and similar circuits, such as current steering DACs operating in multiple phases, is inefficiency. In the case of a switched-capacitor integrator, the current drive capability of the operational amplifier is only exploited approximately half of the time for a two-phase design. In other words, while the operational amplifier does provide current drive during the dump phase, its current drive capability is generally not used during the sampling phase.
Addressing the problem of circuit inefficiency is major effort, especially in delta-sigma modulator applications. Among other things, an improvement in circuit efficiency can result in a tradeoff of other performance parameters, such as noise attenuation. Hence, some improved techniques are required for designing and constructing efficient multiple-phase filters and associated delta-sigma modulators that do not sacrifice noise performance or other operating characteristics.
The principles of the present invention are embodied in circuits and methods for performing delta-sigma modulation with multiple attenuation bands in the noise transfer function. According to one particular embodiment, a noise shaper is disclosed which includes a filter system for generating a first set of poles and zeros characterizing noise attenuation in a signal baseband of a noise transfer function and at least one additional set of at least one pole one and zero characterizing noise attenuation in at least one additional band outside the baseband of the noise transfer function.
Multiple attenuation bands in the delta-sigma modulator noise transfer function realize significant advantages. For example, a delta-sigma modulator with n number of attenuation bands defined on the unit circle in the z-plane will allow an output signal to be interleaved into n number of conversion elements. The noise attenuation in the multiple attenuation bands ensures that noise, which would otherwise be demodulated by mismatches between the interleaved conversion elements, is minimized. In the case of a switched-capacitor DAC or summer, interleaved conversion elements in turn allow the current capability of the output operational amplifier to be fully exploited in n number of non-overlapping phases. In a current steering circuit, such as a current steering DAC, interleaved current steering elements provide for the generation of a smoother output signal.